Field of the Invention
The present invention relates to a robot system including a multi joint robot and a camera. More specifically, the present invention relates to a method of determining a rotational axis offset of multiple joints of a multi joint robot.
Description of the Related Art
Multi joint robots in which both ends of link units are coupled by pivotal joints to allow the link units to bend have been widely used. The multi joint robots typically include torsional joints that allow some of link units to rotate in cross sections perpendicular to the longitudinal directions of the link units in torsional directions. Accordingly, the multi joint robots can perform complicated movement, such as six-axis motion or seven-axis motion, which is similar to the movement of human arms.
The multi joint robot may interfere with another object during installation, teaching or actual movement to cause an overload on a certain joint; the joint, in turn, causes an origin offset (rotational axis offset) at the joint. Actual movement in a state where the certain joint has the origin offset may possibly reduce movement accuracy, cause a jig or a workpiece to fall off, and cause an interference with another object that results in an overload.
Japanese Patent Application Laid-Open No. 563-288696 describes a robot system that includes a multi joint robot and a camera; as to the robot, a base unit and two links are coupled by two joints. Here, an end effector of the multi joint robot is provided with a mark. The mark found during actual movement is photographed by a television camera to acquire the moving trajectory of the mark. If the moving trajectory deviates, the movement of the robot system is stopped.
Japanese Patent Application Laid-Open No. 2011-125976 describes a robot system that includes a multi joint robot and a camera; as to the robot, a base unit and three links are coupled by three joints. Here, marks having specific shapes are provided at positions adjacent to the respective joints of the multi joint robot. The marks photographed by the camera are image-analyzed to measure the pivotal positions of the respective link units.
In a conventional multi joint robot, a pinhole is formed at an overlapping portion of two link units sandwiching a joint so as to allow a pin to be inserted through the pinhole in the case with no origin offset. By a manual operation that returns the respective joints to the origins and inserts the pins into the pinholes, determination is made as to whether an origin offset occurs or not for each joint individually. Evaluation of the origin offsets of many joints of the multi joint robot thus requires a skilled technician to take a long time.
The robot system in Japanese Patent Application Laid-Open No. S63-288696 can immediately detect occurrence of an origin offset of a joint by means of an offset of the moving trajectory of a mark in a photographed image. However, another manual operation is required to determine which joint causes the origin offset.
The robot system in Japanese Patent Application Laid-Open No. 2011-125976 can determine presence or absence of an origin offset at each joint through a photographed image to a certain extent. However, measurement of the origin offsets of multiple joints through one camera cannot highly accurately detect the origin offsets of the respective joints. Installation of cameras for the respective marks can increase accuracy in detecting the origin offset of each joint. However, installation of many cameras around the multi joint robot having a high degree of freedom is economically unfavorable. Furthermore, it is difficult to prepare positions where the cameras are to be installed.